Such a satellite is part of a global satellite-aided positioning system also known by the English abbreviation GNSS (for “Global Navigation Satellite System”).
In a general manner, a GNSS consists of a plurality of satellites that make it possible for a portable receiver to determine its position in a terrestrial reference frame, as well as its speed and time related information.
There are currently several GNSS based systems in existence, among which mention may be made in particular of the GPS, the GLONASS, or even the GALILEO which is expected to be put in operation in the near future.
The satellites of such a GNSS are capable of transmitting electromagnetic signals comprising in particular a navigation related information element.
Each navigation related information element generally includes data related to the transmission time for transmission by the satellite of the corresponding signal and to the current position of the satellite. In particular, the data related to the current position of the satellite generally contain the almanac giving a rough position of the satellite and the ephemerides giving the exact current position of the satellite.
The navigation related information element is carried by a carrier wave and modulated by a spreading code that is specific to each satellite. Thus, the signals are transmitted by the satellites using a spread spectrum technique.
The receiver is capable of receiving the signals transmitted by the satellites and of extracting the navigation related information element in order to in particular determine the distance to the satellite that has transmitted the corresponding signal. This distance, also known as pseudo-range, is determined by analyzing the propagation time for propagating the corresponding signal.
In order to determine its position, speed and the timing information, the receiver effectively deploys the digital treatment processing of the navigation related information elements from at least three different satellites.
In practice, in order to obtain a more precise position, the receiver needs navigation related information elements originating from at least four different satellites.
More precisely, in order to acquire the navigation related information from a given satellite, the receiver carries out two phases that process the signals coming from this said satellite.
During an initial phase, called the acquisition phase in the state of the art, the receiver generates a local signal containing in particular a local spreading code that presents the image of the spreading code of the satellite.
As initially the receiver does not know its position, the local signal is not synchronized with the signal received. This means in particular that the local signal is carrier frequency offset from the received signal by a value known as the Doppler value, and that the spreading code of the received signal is time lagged from the local spreading code by a value known as the lag value.
Then the receiver carries out a search for a peak of the correlations between the local signal and the received signal by trying various different Doppler values and lag values.
When a peak is detected, the receiver determines the Doppler and lag values corresponding to this peak and based on these values, launches a subsequent phase, called the continuation phase in the state of the art.
During the continuation phase, the receiver regularly updates the Doppler and lag values, and extracts the navigation related information element from the signal transmitted by the satellite using in particular the local spreading code and the Doppler and lag values determined.
At the end of the acquisition phase, it is considered that the receiver is synchronized with the satellite or even “locked in” to the satellite.
This means in particular that the receiver was able to find the Doppler and lag values related to this satellite so as to initiate the continuation phase.
It sometimes happens that the receiver synchronizes its local signal corresponding to the desired satellite on the received signal from another satellite, which leads to an erroneous measurement of distance, and therefore potentially to a wrong positioning.
In this event, it involves a wrong synchronization or a wrong “locking in”.
This comes about for example when the correlation between the local signal and the signal received from the desired satellite gives less power than the correlation with the signal received from another satellite, due to a high deviation in received power.
There exist in the state of the art various different methods that make it possible to avoid such a wrong synchronization.
Thus, a method used in the conventional way, consists of checking and verifying the consistency between the satellite position calculated based on the ephemerides contained in the navigation related information element and the one calculated based on the almanac, which contains the identifiers of the satellites, in contrast to the ephemerides. The inconsistency between these values thus signifies a wrong synchronization.
However, this method is not completely satisfactory. In particular, it necessitates collecting all of the ephemerides contained in the navigation related information element which is a relatively long process. In practice, this could take up to two minutes.
It is then conceivable that this penalizes the operation of the receiver, particularly following a masking of the satellite, and does not provide ensuring a continuity of service.
The present invention aims at overcoming this drawback.